Add a simple script for freezing package version constraints

Also created tools/ for various tools used when maintaining CMS.

Author: gollux

tools/freeze-constraints.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+set -e
+
+(
+	echo "# Constraints generated by $0"
+	pip freeze | grep -v '^cms'
+) >constraints.txt